A wide variety of optical measurement instruments are now available to characterize one of more parameters of a subject's eye. On such optical measurement instrument is an aberrometer, which may be employed to obtain objective refraction measurements of a subject's eye. An aberrometer may include a Shack-Hartmann wavefront sensor, which may measure the values for one of more refraction orders of a subject's eye from a light wavefront which is returned from the eye.
Aberrometers typically measure the refraction of a subject's eye using infrared light.
However, in general a physician or clinician wants to know the refraction of a subject's eye with visible light. So a chromatic adjustment must be made between the infrared light using for the measurements and the visible light for which the refraction is sought. In this patent application, we will refer to such an adjustment as a “Chromatic Adjustment Factor (CAF).”
Typically, an optical measurement instrument which includes as an aberrometer or autorefractor simply subtracts 0.7 diopters from the infrared refraction to get the visible refraction. That is, it applies a CAF of 0.7. A CAF of 0.7 brings the average calculated visible refraction into agreement with the average manifest refraction over a large sample of subject eyes.
However, tests indicate that for about twenty percent of subjects, the refraction for the subject in the visible spectrum which is obtained by applying a standard CAF of 0.7 to the value measured by the instrument using infrared light will be have a difference or “error” of more than 0.5 diopters from the correct value for that eye. Eye glasses prescribed with such an error will result in complaints from many subjects. It is believed that this error is a product of the fact that the correct CAF which should be applied varies from eye to eye, depending on differences between subjects in parameters such as the change in the index of refraction of the cornea from the infrared wavelength to the visible wavelength, the change in the index of refraction of the lens from the infrared wavelength to the visible wavelength, etc.
Accordingly, it would be desirable to improve the CAF value used with aberrometer measurements so that the calculated visible refraction matches better to the manifest refraction that an optometrist would determine with a phoropter.